A new and improved power steering apparatus includes valve and spring assemblies which are effective to compensate for differences in size of operating surface areas of a power steering motor.
In known power steering systems, a piston and cylinder type fluid motor is connected with a rack gear. Upon actuation of a valve assembly, fluid pressure is ported to either the head end or the rod end of the piston to effect operation of the motor to assist in turning a steerable vehicle wheel. In these known power steering systems, the piston rod does not extend completely through the power steering motor so that the head end surface area of the piston is greater than the rod end surface area. Therefore, if the same fluid pressure is applied against the head end surface area during turning movement in one direction and against the rod end surface area during turning movement in the opposite direction, a greater force is applied to the rack gear during turning movement in the direction in which fluid pressure is applied against the head end surface area.
With these known power steering systems, the resistance of the power steering control valve to actuation in either direction is the same if the valve is moved through the same distance. Therefore, the feel or resistance which is transmitted back to an operator of the vehicle is the same when the power steering valve is actuated through the same distance to effect a turn in either direction. This is true even though the output force of the power steering motor is different in one direction than the other due to the difference in the size of the head and rod end surface areas. Typical of these known power steering systems are the ones disclosed in U.S. Pat. Nos. 3,709,099 and 3,303,051.
In a power steering system disclosed in U.S. Pat. No. 3,393,608, a torsion bar is utilized to provide feel to an operator upon actuation of a valve assembly. Clockwise rotation of a steering wheel is effective to twist one-half of the torsion bar. Counterclockwise rotation of the steering wheel is effective to twist only the other half of the torsion bar. The two halves of the torsion bar have equal resistance to turning movement.
Upon rotation of a steering wheel in one direction, fluid pressure is ported to a relatively large head end surface area of a piston by the steering control valve assembly disclosed in the aforementioned U.S. Pat. No. 3,393,608. Upon rotation of the steering wheel in the opposite direction, fluid pressure is ported to the relatively small rod end surface area of the piston. Although the torsion bar utilized in this patent provides equal resistances to actuation of the valve in opposite directions, the output force from the power steering motor is not the same in each direction since the head end surface area of the piston is larger than the rod end surface area.
In order to compensate for the unequal head and rod end surface areas of the piston of a power steering motor, U.S. Pat. No. 2,757,748 suggests that a power steering control valve have reaction surface areas which are sized to be proportional to the head and rod end surface areas of the piston of the power steering motor. Fluid pressure directed against the reaction surface areas of the valve is effective to apply a force to the valve which varies as a function of the force provided by the power steering motor. However, the use of hydraulic pressure against different size surface areas of a valve results in a relatively complicated valve structure which may be difficult to fabricate and may be unsuitable for use with many different types of power steering systems.